Neutron tubes of this kind are used in techniques for the examination of substances by means of fast, thermal, epithermal or fast cold neutrons: neutronography, analysis by activation, analysis by spectrometry of the inelastic diffusions or radiative captures, diffusion of neutrons etc.
In order to make these nuclear techniques as effective as possible, longer tube service lives are required for the corresponding emission levels.
The fusion reaction d(3.sub.H, 4.sub.He)n which supplies 14 MeV neutrons is most commonly used because of its large effective cross-section for comparatively low ion energies. However, regardless of the reaction used, the number of neutrons obtained per unit of charge in the beam always increases in proportion to the increase of the energy of the ions directed towards a thick target, that is to say mainly beyond ion energies obtained in sealed tubes which are available at present and which are powered by a high voltage which does not exceed 250 kV.
Erosion of the target by ion bombardment is one of the principal factors restricting the service life of a neutron tube.
The erosion is a function of the chemical nature and the structure of the target on the one hand, and of the energy of the incident ions and their density distribution profile on the surface of impact on the other hand.
In most cases the target is formed by a hydride (titanium, scandium, zirconium, erbium etc.) which is capable of binding and releasing large quantities of hydrogen without substantially affecting its mechanical strength; the total quantity bound is a function of the temperature of the target and of the hydrogen pressure in the tube. The target materials used are deposited in the form of thin layers whose thickness is limited by problems imposed by the adherence of the layer to its substrate. One way of restarting the erosion of the target, for example is to construct the absorbing active layer as a stack of identical layers which are isolated from one another by a diffusion barrier. The thickness of each of the active layers is in the order of magnitude of penetration depth of deuterium ions striking the target.
Another method of protecting the target, thus increasing the service life of the tube, consists in the influencing of the ion beam so as to improve its density distribution profile on the surface of impact. For a constant total ion current on the target, leading to a constant neutron emission, this improvement will result from an as uniform as possible distribution of the current density across the entire target surface exposed to the ion bombardment.
In a sealed neutron tube the ions are generally supplied by a Penning-type ion source which offers the advantage that it is robust, has a cold cathode (and hence a long service life), supplies large discharge currents for low pressures (in the order of 10 A/torr), and has a high extraction yield (from 20 to 40%) with small dimensions.
This type of source, however, has the drawback that it requires the use of a magnetic field in the order of a thousand gauss, parallel to the axis of the ionisation chamber, which introduces a substantial transverse inhomogeneity of the density of the ion current inside the discharge and at the level of the extraction taking place along the common axis of the field and the source.
Another drawback is due to the fact that the ions extracted and accelerated towards the target react with the gas molecules contained in the tube at a constant pressure of the first order in order to produce ionisation, dissociation and charge exchange effects which cause on the one hand a reduction of the energy on the target, that is to say to a reduction of the production of neutrons, and on the other hand the formation of ions and electrons which are subsequently accelerated so as to bombard the ion source or the electrodes of the tube.
This results in energy depots which increase the temperature of the electrode material such as molbydenum or stainless steel. The heating of these materials causes desorption of impurities such as carbon oxide enclosed thereby, thus deteriorating the quality of the tube atmosphere. The ions of impurities formed in the tube, for example Co.sup.+, bombard the target with a pulverisation coefficient which is a factor from 10.sup.2 to 10.sup.3 higher than that of the deuterium-tritium ions, thus substantially increasing to the erosion. These effects are greater as the operating pressure in the neutron tube is higher.